The process for creating a well (e.g., oil, natural gas) typically includes drilling a hole through a ground formation, inserting a well casing into the hole, and filling the resulting annulus between the interior surface of the drilled hole and the exterior surface of the well casing with a suitable sealing medium (e.g., cement). The sealing medium serves to prevent flow of a liquid (e.g., oil) and/or a gas (e.g., natural gas) along the annulus so as to inhibit and preferably prevent leakage of the liquid and/or gas via the annulus. Typically, the sealing medium is injected into the annulus at an end of a length of the well casing and flows longitudinally along the annulus to fill the annulus along the length of the well casing.
Unfortunately, the injection of the sealing medium may sometimes result in an annulus that is inadequately sealed. Because an inadequately sealed annulus can result in significant detrimental leakage along the annulus, it is important that any inadequately sealed portions of the annulus be identified and fixed. Ultrasonic inspection is typically used to evaluate if the annulus is adequately sealed. An ultrasonic inspection tool is moved along the inside of the well casing, transmits an ultrasonic pulse towards the inside wall of the well casing, and generates an output signal in response to the returning ultrasonic waves. Analysis of the output signal is performed to evaluate, in part, the acoustic impedance of the sealing medium in the annulus. The acoustic impedance of the sealing medium can then be assessed to evaluate if the annulus at the inspected location is adequately or inadequately sealed. The process is repeated along the depth of the well casing and repeated at different azimuth angles to sufficiently inspect the annulus.